A challenge in developing high power III-V material-based semiconductor devices, such as light emitting diodes (LEDs), laser diodes (LDs), bipolar junction transistors (BJTs), and heterojunction bipolar transistors (HBTs), is the development of an ohmic contact that has both a low specific resistance and a high current carrying capability. For example, the challenge to manufacture a low resistance ohmic contact to n-type material is particularly important for deep ultraviolet LEDs made from group III-nitride materials, such as Aluminum Gallium Nitride (AlGaN) or Aluminum Gallium Indium Nitride (AlGaInN), which include a high molar fraction of aluminum.
To achieve a low n-type contact resistance in a nitride-based device, several contact metals and a relatively high annealing temperature are generally used. To this extent, Al can be used as a contact metal because of its relatively low melting point of approximately 660 degrees Celsius. Furthermore, Titanium (Ti) or Chromium (Cr) can be used as the first layer of the contact due to their low metal work function to nitrides. Specific examples include Ti/Al/Ti/Gold (Au) or Ti/Al/Nickel (Ni)/Au, with thicknesses from five nanometers to five microns and which are annealed at 400 degrees Celsius or higher temperatures. Another approach reverses the order of the Ti and Al, and forms an Al/Ti-based contact to an n-type GaN semiconductor, which includes Al/Ti/Platinum (Pt)/Au and which is annealed at temperatures between 400 and 600 degrees Celsius. Still other approaches form a Cr/Al-based contact to an n-type GaN semiconductor, which include various metal configurations, such as Cr/Al/Cr/Au, Cr/Al/Pt/Au, Cr/Al/Pd/Au, Cr/Al/Ti/Au, Cr/Al/Cobalt (Co)/Au, and Cr/Al/Ni/Au.
Contact reliability also can be a problem. For example, to date, Ti/Al-based n-type contacts for ultraviolet LEDs emitting 265 nanometer and shorter wavelengths have not been shown to be very reliable.
Some approaches have improved an ohmic contact through re-growing semiconductor layers after performing the etching process. For example, in one approach, a non-alloyed contact is formed through a process of re-growing semiconductor layers. The process includes: (1) growing semiconductor layers on a substrate such as sapphire; (2) disposing a regrowth mask above the top semiconductor layer, where the regrowth mask material (e.g., silicon nitride or silicon dioxide) is chosen, deposited, and selectively removed (e.g., through the use of a photoresist) so that is can function as a passivation layer on the semiconductor surface; (3) etching the semiconductor layers with an acceptable depth being approximately five to one thousand nanometers past the surface of the top semiconductor layer; (4) growing structures in the etched regions; and (5) optionally applying photolithography to define a gate region for devices that include gates.
FIGS. 1A-1C show typical ohmic contacts 2A-2C, respectively, formed using reactive ion etching (RIE) and re-growth processes according to the prior art. In each case, RIE of a semiconductor layer 4A-4C is employed prior to re-growth. Re-growth is carried through with subsequent deposition of an ohmic metal 6A-6C. In FIG. 1A, the ohmic contact 2A includes a regrown surface state compensating layer on the RIE damaged layer 4A. In FIG. 1B, the ohmic contact 2B includes a regrown surface state compensating layer and a delta doping layer. In FIG. 1C, the ohmic contact 2C includes a regrown surface state compensating layer and a layer for low specific ohmic contact resistance. As illustrated in FIGS. 1A-1C, the regrown technique can be used in conjunction with other features, such as delta-doping as shown in FIG. 1B. The delta-doping has significance for high speed devices, such as heterostructure field-effect transistors, which employ the delta-doping technique to achieve a high carrier density, and large breakdown voltage of the gate. In addition, the regrowth technique can be used together with contacts that include a rough morphology as shown in FIG. 1C, as well as contacts that are annealed with regrowth regions and have protrusions into regrowth region. While the quality of each of the contacts 2A-2C is significantly improved by re-growing as compared to direct deposition of the ohmic metal 6A-6C, each process has a large number of defects in the RIE etched region 4A-4C, which reduces the quality of the ohmic contact 2A-2C and decreases an overall lifetime of the corresponding device including the ohmic contact 2A-2C.
Another regrowth approach is specifically designed to regrow group III nitride semiconductor layers. The process includes: (1) growing a semiconductor body on a substrate including semiconductor layers with an unintentionally doped (UID) gallium nitride (GaN) layer overlying the semiconductor layers and a UID aluminum gallium nitride (UID-AlGaN) layer overlying GaN semiconductor layers; (2) depositing and patterning an insulating film; and (3) re-growing an n+ GaN layer at regions of the surface of the UID-AlGaN not covered with insulating film without etching the surface of the UID-AlGaN semiconductor.